This invention relates to tuning indicators for radio receivers and, more particularly, to the provision of a single indicator and simplified circuitry to indicate both station tuning and stereo signal conditions.
Tuning indicators have a long history in the field of broadcast receivers, both AM and FM. Various kinds of devices have been utilized; in some receivers meters have been used; "tuning eye" or electron ray tubes were popular for a time, but generally an incandescent lamp bulb was coupled to a circuit which would only be activated when a station was properly tuned in. At the present time, many receiver designs employ light-emitting diodes (LED's) as indicators.
In FM stereophonic broadcasting, the stereo presence signal or pilot tone is a 19 kHz signal which is modulated onto the carrier only when the station is transmitting in stereo. In a receiver, a local oscillator at 78 kHz is divided down to provide the 38 kHz suppressed carrier of the stereo (L-R) signals. An LED may be used to indicate the presence of stereo signals and, typically, the LED circuit is coupled to the phase locked loop which is locked to the 19 kHz pilot tone.
In AM stereophonic broadcasting, on the other hand, the pilot tone is a subaudible or nearly subaudible tone which is added to the (L-R) signal before the latter is modulated onto the carrier, by whatever modulation scheme is desired. This low frequency tone is detected along with the L-R signal, then separately detected for use in activating the stereo indicator and, typically, a mono/stereo mode switch. Such a mode switch is desirable since operation of the stereo (L-R) circuits during monophonic reception can denigrate the performance of the receiver.
Presently, some AM stereo receivers utilize one indicator lamp or LED for indicating "locked-in" tuning, and a second lamp or LED to indicate that the station is broadcasting in stereo. In the interest of simplicity of design and lower cost, particularly in the case of lower priced manually tuned receivers, it would be desirable to provide, with one simple indicator, an indication that the receiver is properly tuned to a station and also whether that station is broadcasting in stereo.